Fluid filled pressure transducer



Oct. 31, 1967 H. L. PASTAN FLUID FILLED PRESSURE TRANSDUCER Filed June9, 1965 so m TN M n o mm l r 7 4 8 2 OVS H V k 7 7 8 BNA w Q IP 74 2 L MW ll 4 1 WK .J .....v. O H fiv// 6% m 7 R mE M U v Y 7 3% 3W v 8 i. .8 U6 2 3 w. 8 8 6 5 m2/w2 m Z 2 m 4 X lw fl wwnww United States Patent3,349,623 FLUID FILLED PRESSURE TRANSDUCER Harvey L. Pastan, Brookline,Mass, assignor to Abex Corporation, a corporation of Delaware Filed June9, 1965, Ser. No. 462,542 Claims. (Cl. 73--398) ABSTRACT OF THEDISCLOSURE A pressure transducer with a coupler at one end and a sensorat the other connected by a capillary tube. Volumetric displacement isminimized by limiting the total liquid fill to the order of .004 cubicinch.

This invention relates to pressure transducers and more particularlycomprises a new and improved fluid filled direct sensing pressuretransducer.

Fluid filled direct sensing pressure transducers are designed to be usedin those systems in which it is undesirable for the medium whosepressure is being measured to enter into the instrument. In such casesthe instrument itself is filled with a fluid which is coupled by meansof a diaphragm or some other device to the medium whose pressure is tobe measured, and the fluid which fills the instrument directly transmitsthe pressure of the medium to the sensing device. The fluid fill mustnecessarily have a boiling point higher than the boiling temperature ofthe medium to which the instrument is exposed, as the introduction ofvapor pressure into the gage will produce extraneous readings at thesensing device which are not representative of the pressure beingmeasured. Similarly, the freezing point of the fluid should be lowerthan the minimum operating range temperature.

One important object of this invention is to provide a fluid filledpressure transducer providing full scale deflection of the sensingdevice with a minimum volumetric displacement. The minimum volumetricdisplacement of the fluid filling the instrument will minimize theeffect upon the fluid medium whose pressure is being measured. Theminimum volumetric displacement necessarily results in a minimum couplerdeflection, and if the coupler is a diaphragm, minimum deflection willmaintain the diaphragm deflection within the linear range. Diaphragmdeflection should not exceed 3% of the diaphragm diameter to maintainthis linear operation.

Another important object of this invention is to provide a fluid filledpressure transducer which dissipates the eifect of fluid expansion. Ifthe eifect of fluid expansion is not dissipated, it will causedisplacement of the sensing device, which in turn will render anextraneous pressure measurement.

Yet another important object of this invention is to provide a fluidfilled pressure transducer which is capable of sensing fluid pressuresat relatively inaccessible locations.

To accomplish these and other objects, the pressure transducer of thisinvention includes a frame having a capillary tube extendingtherethrough which at one end communicates with a chamber defined inpart by the coupler. The other end of the capillary tube communicateswith a strain gage type sensing device which has very small deflectionthroughout its full operative range. A liquid fills the sensing device,chamber and capillary tube so as to directly transmit the pressureapplied against the diaphragm to the sensing device to render a pressuremeasurement. In accordance with one embodiment of this invention, atemperature sensing capsule is disposed adjacent the diaphragm, whichforms part of a separate temperature compensating system to balance theeffect upon the sensing device of expansion of the liquid which fillsthe chamber and capillary tube.

These and other objects and features of this invention along with itsincident advantages will be better understood and appreciated from thefollowing detailed description of two embodiments thereof, selected forpurposes of illustration and shown in the accompanying drawing, inwhich:

FIG. 1 is a cross-sectional view of one form of fluid filled pressuretransducer constructed in accordance with this invention;

FIG. 2 is a cross-sectional view of another embodiment of fluid filledpressure transducer constructed in accordance with this invention; and

FIG. 3 is a schematic diagram illustrating the strain gage circuitemployed in each of the transducers shown in FIGS. 1 and 2.

The embodiment of this invention shown in FIG. 1 includes a main body10, a sensing device 12 provided at one end of the body 10, a capillarytube 14 which extends through the body 10, and a coupler '16 secured toand closing the other end of the body. 7

The body 10 has an upper generally cylindrical portion 18 provided withan annular recess 20 formed in its outer surface 22 and a stem 24 whichextends from the upper portion 18 toward the diaphragm coupler 16. Theupper body portion 18 is surrounded by a sleeve 26 that encloses theshallow annular recess 20 so as to define with it a closed pressurechamber 28. The wall 30 of the sleeve 26 is relatively thin and flexibleand therefore is capable of barrelling outwardly in response to pressurein the chamber 20. The ends 32 and 34 of the sleeve 26 are welded orotherwise secured to the adjacent ends 36 and 38, respectively, of theupper body portion 18 so as to prevent elongation of the sleeve underpressure.

The upper body portion 18 and the sleeve 26 along with their incidentparts described above, define the sensing device 12 for the gage. Itwill be noted that strain gage windings 40 are bonded circumferentiallyabout the outer surface of the wall 30 of the sleeve 26, which sense thedistortion of the wall 30 under changes of pressure within the chamber28.

The chamber 28 of the sensing device 12 communicates through a passage42 within the upper body portion '18 with the upper end 44 of thecapillary tube 14 imbedded in the body 10. The capillary tube 14 extendsdownwardly through the stem 24 of the body, a protective jacket 46 and alower stem 48 to a small chamber 50 enclosed by the coupler 16.

The protective jacket 46 may either be rigid or flexible depending uponthe particular use for which the transducer is designed. If the coupler16 is to be exposed to fluid mediums in relatively inaccessible pointsin a system, some advantage may be derived from making the jacket 46 outof flexible material such as a wire-type helical cable, and making thecombined length of the stem and jacket relatively long.

The diaphragm coupler 16 is relatively flexible and has a low springrate. It is subject to minimum stresses as it is completely supported onthe inside by the fluid 52 which fills the chamber 50. Unlike diaphragmswhich are connected to and work against push-rods, strain tube-s orother similar devices in unfilled instruments, which diaphragms operateat high stress and are subject to diaphragm rupture, the diaphragm ofthis invention operates at very low stress levels because it issupported on the back side by substantially the same pressure which isexerted against the outside or exposed face. The fluid 52 which fillsthe chamber 50 also fills the capillary tube 14, passage 42 and annularchamber 28 forming part of the sensing device.

In FIG. 1 it will be noted that the windings 40 are woundcircumferentially about the sleeve 26. The windin gs form two legs ofthe bridge circuit 56 shown in FIG. 3. The other two legs of the bridgecircuit com-prise the dummy windings 58 wound about the block 60 securedto the upper end 36 of the upper body portion 18. It is apparent thatchanges in pressure of the fluid in the chamber 28 will cause the wall30 of the sleeve 26 to barrel outwardly, and this distortion will besensed by the active arms 40 of the strain gage bridge circuit shown inFIG. 3, and by means of the different leads connected to the bridge, thebridge may be energized and a reading may be made at a remote locationwhich is indicative of the change in pressure within the chamber 28.

When the transducer is used in a temperature range having a maximum ofapproximately 600 F. mercury is the ideal fluid to fill the couplerchamber 50, the capillary tube 14, passage 42 and annular chamber 28.Mercury has a low compressibility and a boiling point in excess of 600F. so that within that operative range no vapor pressures will begenerated to cause secondary expansion of the chamber 28 to distort thewall 30 of the sleeve 26.

It was suggested in the introduction that it is extremely desirable thatminimum volumetric displacement occur over the full operative range ofthe transducer. In order to provide the reader with an appreciation ofthe size of the transducer and the volumetric displacement, some typicaldimensions are offered. In the preferred form of this invention, thecapillary tube 14 has an inner diameter of approximately .010 inch, andthe sensing chamber 28, defined by the gap between the inner surface ofthe sleeve 26 and the outer surface of the body portion 18 at the recess20, is .005 inch. The chamber 50 is approximately .010 inch deepmeasured from the inner surface of the diaphragm 16, and the capacity ofthe annular chamber 28, passage 42, capillary tube 14, and chamber 50 isonly approximately .004 cubic inch.

In FIG. 1 it will be noted that a section of the lower stem 48 isexternally threaded as suggested at 62 to facilitate the mounting of thetransducer in the wall of a chamber containing the medium whose pressureis to be meas ured. It is evident that when the lower stem 48 is screwedinto the wall, the very short section 64 may extend through the wall. Itis important in certain applications that the section which extendsthrough the wall cause a mini-mum disturbance of the flow of the mediumwhose pressure is being measured. For example, when the transducer isused to measure the pressure of a plastic melt, the portion 64 shouldnot appreciably interrupt the flow of the plastic, because aninterruption in the flow may cause the plastic to deteriorate.Consequently it is important to employ a very small diaphragm 16 so thatthe diameter of the section 64 which may project into the medium isrelatively small. Because the diaphragm 16 is small, and itsdisplacement is necessarily small to remain within the linear range, thetotal volumetric displacement of the liquid which fills the transduceris also small. The small volumetric displacement of the liquid require-sthat a sensing device be used which also has a small volumetricdisplacement over its full operative range. Therefore, such sensingdevices as Bourdon tubes which have a relatively large volumetricdisplacement should not be used in the device. Rather, thestrain-gage-type capsule sensing device is used which does not elongateand which experiences only a small volumetric displacement over its fulloperative range.

By making the diaphragm 16 extremely soft, the diaphragm will dissipatea substantial portio of the eifects of expansion of the liquid whichfills the transducer. It is important that the expansion of the liquidcaused by such conditions as temperature cause a distortion of thediaphragm 16 rather than a distortion of the wall 30 of the sensingdevice. If the expansion causes a distortion in the sensing device anextraneous reading will be produced at the bridge circuit which will notsolely represent changes in pressure on the fluid.

In the embodiment shown in FIG. 2 a temperature compensating subassemblyis incorporated into the device which may otherwise be the same as theembodiment of FIG. 1. This subassembly is particularly desirable whenthe transducer is designed to operate over a wide temperature range andthe diaphragm coupler is unable to wholly dissipate the effects ofliquid expansion. The transducer shown includes a body 70 very similarto the body 10 of the embodiment of FIG. 1 which at its upper enddefines a sensing device 72. The sensing device 72 may be identical tothe sensing device 12. The annular chamber (not shown) of the sensingdevice is connected by a capillary tube 74 to the chamber 76 at thebottom of the lower stem 78 enclosed by the soft diaphragm 80. Thus,when the diaphragm 80 is exposed to a pressurized medium, that pressureis transmitted through the liquid 82 which fills the chamber 76,capillary 74 and the annular chamber (not shown) in the sensing device.

Unique to this embodiment is the temperature compensating subassembly 84which includes a second capillary tube 86 having a temperature bulb 88at its lower end 90 within the chamber 76. The capillary tube 86 extendsthrough the sensing device 72 and its upper end 92 communicates with apassage 94, in the second sensing device 96. The sensing device 96 issubstantially identical to the sensing device 72 and includes an annularchamber 98 surrounded by a thin-walled sleeve 100 which in turn carriesthe strain gages 102. The strain gages 102 are connected in a bridgecircuit with the gages 104 about the sensing device 72. Thus, unlike thestrain gage circuit of the embodiment of FIG. 1, the circuit of theembodiment of FIG. 2 has four active legs.

The bulb 88, capillary 86, passage 94 and annular chamber 98 are filledwith the same fluid which fills the chamber 76, capillary 74 and annularchamber of the sensing device 72. Thus, the liquid in the temperaturecompensating subassembly 84 experiences the same expansion as does theliquid in the capillary 74 and annular chamber of the sensing device 72.However, because the bulb 88 is closed and nonyielding, the liquid whichfills the temperature compensating subassembly is not subjected tochanges in pressure through the diaphragm coupler 80. Rather, changes inpressure are experienced only by the sensing device 72. By connectingthe four strain gage windings as shown in FIG. 3, it will be appreciatedthat the four active legs will cause any distortion of the gages 104 dueto expansion of the liquid to be canceled by the correspondingdistortion of the gages 102 in the temperature compensating subassembly.Because the diaphragm 80 is soft, care should be taken in the embodimentof FIG. 2 not to overcompensate for temperature effects. To avoidovercompensation, it may be necessary to use a thicker-walled sleeve100, or less sensitive strain gages 102 in the sensing device 96 thanfor the windings 104, because certain of the effects on windings 104will be lessened due to the flexibility of the diaphragm 80.

From the foregoing description those skilled in the art will appreciatethat numerous modifications may be made of this invention withoutdeparting from its spirit. Therefore, it is not intended to limit thebreadth of this invention to the embodiments illustrated and described.Rather, it is intended that the breadth of this invention be determinedby the appended claims and their equivalents.

What is claimed is:

1. A pressure transducer comprising an elongated frame,

a capillary tube extending through the frame and terminating at one endadjacent one end of the frame, said tube having an inner diameter in theorder of .010 inch,

a coupler closing that end of the frame and defining with the frame achamber in communication with the capillary tube,

a liquid filled deformable sensor secured to the frame at its other endremote from the coupler,

and a liquid filling the capillary tube and the chamber and incommunication with the liquid in the sensor for applying the pressureexerted against the coupler to the sensor.

2. A pressure transducer as defined by claim 1 further characterized bysaid sensor comprising a capsule having a deformable cylindrical wall incontact with the liquid,

and strain gage windings bonded to that wall to measure the pressureapplied to the wall by the liquid.

3. A pressure transducer as defined in claim 2 further characterized bysaid liquid filling the tube, chamber and capsule having a combinedvolume in the order of .004 cubic inch.

4. A pressure transducer as defined in claim 3 further characterized bysaid coupler comprising a soft flexible diaphragm.

5. A pressure transducer as defined in claim 1 further characterized bysaid frame having a bendable section through which the capillary tubeextends from the coupler to the sensor.

6. A pressure transducer comprising an elongated frame, a firstcapillary tube extending through the frame and terminating at one endadjacent one end of the frame,

a coupler closing that end of the frame and defining with the frame achamber in communication with the capillary tube,

a first liquid filled strain gage sensor secured to the frame at itsother end remote from the coupler,

a first liquid filling the capillary tube and the chamber and incommunication with the liquid in the first sensor for applying thepressure exerted against the coupler to the sensor, a second capillarytube extending through the frame,

a temperature sensing bulb connected to the end of the second tube anddisposed adjacent the coupler,

a second liquid filled strain gage sensor connected to the second tuberemote from the coupler,

a second liquid filling the second tube and the bulb and incommunication with the second sensor for applying the elfects of changein temperature of the second liquid to the second sensor,

and a bridge circuit made of the strain gages of each sensor.

7. A pressure transducer as defined in claim 6 each of the sensorscomprising a capsule having a deformable cylindrical wall in contactwith the liquid and strain gage windings bonded to the wall.

8. A pressure transducer as defined in claim 6 further characterized bysaid coupler comprising a soft flexible diaphragm.

9. A pressure transducer as defined in claim 6 further characterized bysaid frame having a bendable section through which the capillary tubesextend from the sensors to the coupler and sensing bulb.

10. A pressure transducer as defined in claim 9 further characterized bysaid first liquid and the liquid in the sensor having a combined volumein the order of .004 cubic inch.

References Cited UNITED STATES PATENTS 4/1965 Li et a1. 73-395 X 9/1966Pastan 73-406 X

1. A PRESSURE TRANSDUCER COMPRISING AN ELONGATED FRAME, A CAPILLARY TUBEEXTENDING THROUGH THE FRAME AND TERMINATING AT ONE END ADJACENT ONE ENDOF THE FRAME, SAID TUBE HAVING AN INNER DIAMETER IN THE ORDER OF .010INCH, A COUPLER CLOSING THAT END OF THE FRAME AND DEFINING WITH THEFRAME A CHAMBER IN COMMUNICATION WITH THE CAPILLARY TUBE, A LIQUIDFILLED DEFORMABLE SENSOR SECURED TO THE FRAME AT ITS OTHER END REMOTEFROM THE COUPLER, AND A LIQUID FILLING THE CAPILLARY TUBE AND THECHAMBER